A cable-based system can be used to deliver high-definition digital entertainment and telecommunications such as video, voice, and high-speed Internet services from a headend to subscribers over an existing cable television network. The cable television network can take the form of an all-coax, all-fiber, or hybrid fiber/coax (HFC) network.
Generally, analog video signals and digital bit streams representing various services (e.g., video, voice, and Internet) from various digital information sources are received at the headend and converted to radio frequency (RF) optically modulated signals for transmission over the cable network. One or more services output from the headend can occupy a specific 6 MHz-wide RF channel having a center frequency that typically falls within a frequency range having a lower limit of 50 MHz and an upper limit of 1002 MHz.
For digital bit streams, in some implementations, each of the digital bit streams is encoded to produce a corresponding digital QAM symbol stream. Each digital QAM symbol stream is root-nyquist filtered, converted to an analog QAM symbol stream, and QAM modulated onto an RF carrier signal having a frequency that corresponds to a center frequency of a 6 MHz-wide RF channel. For digital broadcast services (e.g., service that are intended for all subscribers in a serving area) such as video, the RF carrier signal frequency typically falls within a frequency range having a lower limit of 550 MHz and an upper limit of 750 MHz. For digital narrowcast services (e.g., services that are intended for a single customer in a serving area) such as video on demand, internet data, and telephony, for example, the RF carrier signal frequency can vary from system to system. Broadcast analog video signals received at the headend are modulated onto an RF carrier signal having a frequency that typically falls within a frequency range having a lower limit of 50 MHz and an upper limit of 550 MHz.
Some or all of the analog single-channel modulated RF carrier signals can be combined to produce an analog multi-channel RF signal. One or more analog optical transmitters convert the analog single-channel modulated RF carrier signals and/or multi-channel RF signals to optically modulated signals.
Generally, the RF optically modulated signals are transmitted from the headend via one or more fibers to one or more fiber nodes. Each of the fiber nodes includes an optical receiver that converts the received optically modulated signals representing broadcast and narrowcast services to electrical RF signals. The electrical RF signals then are transmitted to receiving devices such as cable modems (CMs) and/or settop boxes (STBs) that are served by the fiber node. All of the receiving devices served by the fiber node can receive the electrical RF signals. If the electrical RF signal represents a broadcast service, each receiving device served by the fiber node can process and deliver the corresponding service to the subscriber. If the electrical signal represents a narrowcast service, the receiving device to which the electrical signal is addressed can process and deliver the corresponding service to the subscriber.
In a so-called overlay system, an analog broadcast optical transmitter transmits broadcast RF optically modulated signals on a first fiber. For narrowcast services, for each fiber node there can exist an analog narrowcast optical transmitter in the headend to output narrowcast RF optically modulated signals at a particular wavelength designated for the fiber node. The narrowcast RF optically modulated signals output by an analog narrowcast optical transmitter can comprise one or more RF channels. A multiplexer combines the narrowcast RF optically modulated signals produced by the narrowcast optical transmitters to produce a multi-wavelength RF optically modulated signal on a second fiber. The broadcast RF optically modulated signal transmitted on the first fiber and the multi-wavelength RF optically modulated signal transmitted on the second fiber can be received at an optical transition node (“OTN”). At the OTN, the narrowcast signals are demultiplexed by an optical demultiplexer. For each narrowcast signal output from the demultiplexer, an optical combiner combines the broadcast signal and the narrowcast, and the resulting signal is transmitted to the designated fiber node for delivery to the receiving devices as discussed above.
There is a growing demand for narrowcast services; however, the existing overlay system architecture is not adequate to meet the growing demand for narrowcast services.